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Multiverses - deep ideas clearly explained,
This review is from: The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos (Hardcover)In this book, Brian Green tackles the idea that the universe as we know it is merely one of many - the `multiverse' hypothesis. He describes nine scenarios that lead to this conclusion, including some very speculative ones indeed, such as the suggestion that our universe may not be real, but merely a simulation running on some gigantic cosmic computer. But the two with the most support are one that follows from `inflation', and one based on string theory.
Inflation is the idea that space experienced a super-rapid period of expansion soon after the Big Bang. It is the most popular model that is consistent with cosmological data, especially measurements of the pattern of variations in the microwave background. It implies the existence of parallel universes, each of which itself has parallel universes, leading to a so-called `quilted multiverse'. It follows that there must be universes that are exactly the same as our own, implying that somewhere there is another person exactly the same as you, doing exactly what you are doing right now. As a theoretical speculation, this must rank as the most startling of all.
While accepting that at present string theory has produced no experimentally testable predictions, Greene is more upbeat than many of his particle physics colleagues, the optimism coming from his belief that string theory has in practice unified general relativity and quantum theory, and the contributions that it has made to mathematics. But string theories are far from unique, because of their need for extra dimensions to make them mathematically consistent. Unless a method can be found of specifying the form of these dimensions, there is an almost uncountable large number of possible formulations of the theory, each of which leads to a unique universe with its own physical laws.
String theory contains higher dimensional geometrical objects called `branes' and the existence of these leads to another form of multiverse that differs from the former two because it is accessible only via the extra dimensions of the theory. Another intriguing possibility is a collision between two branes, leading to a situation very similar to the Big Bang and effectively resetting the cosmological clock. This could repeat endlessly, leading to a `cyclic multiverse'. This has the philosophically pleasing feature of avoiding the question of how the universe began - the cycles are endless and so there is no need for an origin of time.
Next Greene returns to the most vexing question in modern cosmology: the origin of the observed (extremely small, but non-zero) cosmological constant. Here he discusses the anthropic principle: the idea that we observe what we do because this is the only environment in which we could exist to make such observations. Curiously, because the argument needs the existence of a vast number of universes to have meaning, the `defect' of string theories having a vast set of possible solutions can be turned to an advantage if each is interpreted as a separate universe - the `landscape multiverse'. While this is not a proof of their correctness, it is a piece of circumstantial evidence in their favour.
The anthropic principle is highly controversial and in another chapter Greene discusses the often made charge that to invoke it is moving into the realm of philosophy, and away from science. The same charge is often leveled against the idea of multiverses in general. Green valiantly counters this view and argues that in principle it should be possible to test some of the predictions of multiverse theories, even if universes other than our own are unobservable. But this relies on being able to calculate the statistical distribution of the physical features of each universe within a multiverse, for example the size of its cosmological constant, which itself rests on a number of unproved, and unprovable, assumptions. He also admits that the calculations needed are still a long way in the future.
The book ends with a philosophical discussion on the limits of inquiry. Here he returns to his controversial plea for a wider interpretation of the meaning of science, beyond the current view that all scientific ideas following from experimentally confirmed phenomena must be testable in principle. He also urges scientists to `take mathematics more seriously' and go where its predictions lead.
Other authors have attempted to give accessible explanations of multiverses and related ideas, but this one is a particularly clear presentation, with an excellent use of everyday analogues, and there is no `dumbing down'. (It is a pity that the quality of the production - rather dark diagrams and small font - does not match that of the text.) As an elucidator of difficult concepts, Brian Greene is one of the very best. Not surprisingly, this book is very thought provoking, but these are deep and complex ideas and the reader must concentrate very hard throughout (a cold towel may also be useful). But even if not everything is fully understood, one will at least have acquired a broad-brush knowledge of current theories in this exciting field.